73 mustang tires

im restoring a 73 mustang, as original as i can make it. I got the factory rims, they're 15x8 inches; i want to put wide tires on the back, but im not sure how wide can that rim take. Im thinking of 275/50 (other post), but im not sure if 8 inch wide rims can use that size of wheel. Do you think i can use that size of wheel, or do i have to use -not so wide- wheels?